A powertrain of a hybrid-electric vehicle includes an internal-combustion engine, an electric motor, a battery pack, and a transmission. The battery pack is connected to the electric motor. The electric motor is coupled to the transmission and transmits rotational kinetic energy to the transmission, and the internal-combustion engine may be coupled to either the electric motor or to the transmission. The transmission transmits the kinetic energy from the electric motor and/or the internal-combustion engine to a drive axle and ultimately to wheels of the vehicle, while applying a gear ratio allowing different tradeoffs between torque and rotational speed.
A hybrid-electric vehicle may include regenerative braking. A regenerative-braking system may include regenerative brakes and friction brakes. The regenerative-braking system applies a ratio of regenerative to friction braking that can vary.
Noise, vibration, and harshness (NVH) constraints can limit the fuel economy possible for the powertrain. More frequently starting and stopping the internal-combustion engine can increase the efficiency of the hybrid-electric powertrain, but the starts and stops may be unpleasant or distracting to occupants. Likewise, reducing the hysteresis between upshifting to and downshifting from a gear can increase the efficiency of the transmission, but more frequent shifting may annoy or distract occupants. Shifting gears based on a state of charge of the battery pack even though desired acceleration is steady can also increase the efficiency of the transmission, but this again leads to more frequent shifting. Increasing the ratio of regenerative to friction braking, for example, by ramping more quickly from regenerative braking to friction braking, can also increase the energy recouped by the regenerative-braking system, but this may produce an uneven braking feel for occupants.